1. Field of the Invention
This invention relates to a process for the sawing of crystal rods or blocks into thin wafers by means of an internal-hole saw in which a cooling lubricant is applied to the cutting edge of the saw blade during the sawing operation.
2. Description of the Prior Art
Internal-hole saws are predominantly used to saw crystal rods or blocks into thin wafers. The saws are especially useful for cutting semiconductor materials, such as silicon, germanium, gallium arsenide or indium phosphide for example, and additionally, those made of oxidic materials, such as gallium-gadolinium-garnet, sapphire, spinel, quartz or glass. These wafers are typically approximately 0.1-2 mm thick and are used, for example, to produce electronic components or solar cells. With internal-hole cutting saws, the sawing operation is performed by a circular rotating saw blade clamped at its outer periphery in a frame. The actual cutting edge of the saw is formed on the inner rim of the saw blade. The cutting edge is provided with a hard abrasive coating which, for example, is normally diamond or boron nitride grains embedded in a nickel layer. During the sawing operation, the workpiece is placed into the circular inner hole and into the cutting position. The cutting edge is then moved through the workpiece until a wafer of the desired thickness is completely severed away. However, if the sawing is performed using the process according to U.S. Pat. No. 4,513,544, a residual joint remains. In these processes, the cooling lubricant is applied to the cutting edge, preferably from supply lines arranged inside the inner hole, wherein the cooling lubricant serves to both conduct away the heat generated during sawing and to carry away the material removed In general, liquids, such as water, which optionally may contain surfactants such as ethylene glycols, organosilanols or highly sulfated fatty acids, or oils based on paraffin, for example, are used as cooling lubricants. These lubricants are usually provided in a supply tank and pumped via a hose system to the point of use and directed via supply lines to the inner and side surfaces of the cutting edge.
It has been found that as a result of forces acting in the saw cut during each sawing operation, the saw blade deviates from the intended cutting line. This results in the wafers obtained always having a generally undesired flexure (often referred to in the industry as "bow" or "warp"). The deviation becomes even more pronounced with increasing service life, and commensurate blade wear until finally, the tolerances specified for flexure of the wafers can no longer be maintained. When this occurs, the cutting accuracy of the cutting edge can be improved, for example, by briefly sawing into a hard material according to German Auslegeschrift No. 2,359,096 or by resharpening the cutting edge. The disadvantage of this procedure is that the deviation of the saw blade from the intended cutting line cannot be established until evidenced by a deficient product, i.e., with the loss of valuable material.
It is known that with the aid of suitable distance measuring devices, such as eddy current measuring instruments, for example, even minimum deviations of the saw blade with respect to a predetermined target position can be recorded during the sawing operation. However, even using these devices, the cutting path cannot be directly controlled and consequently the losses are reduced only slightly if at all. Moreover, it is sometimes necessary to resharpen the saw's cutting edge several times in succession and to check it by means of control cuts until a satisfactory cutting path has again established itself. Additionally, the saw blades cannot be resharpened indefinitely since each sharpening operation reduces their service life. Aside from the time losses involved, resharpening also necessitates frequent changes of the saw blades.